1. Field of the Invention
The present invention relates generally to meters for measuring the velocity of a fluid and more particularly to an acoustic-type flow meter in which basic loop timing is derived from each of a series of ramps.
2. Description of the Prior Art
The operation of fluid-flow-measuring meters of the acoustic type is based upon the principle that the propagation velocity of an acoustic wave in a fluid is equal to the acoustic velocity with respect to the fluid plus the velocity of the fluid. Typically, such meters include a pair of acoustic transducers each adapted for both generating and detecting an acoustic pressure wave in a fluid the velocity of which is to be measured. The transducers are so disposed as to define a communication link therebetween, which extends, at least obliquely, along the direction of fluid flow. The meters transmit an acoustic-wave packet, in turn, in alternate directions across the link while measuring the acoustic propagation period, also referred to as the acoustic time of flight, in both the upstream and the downstream directions. Finally, a difference between the upstream and downstream propagation periods is determined providing a measure of the line integral through a velocity profile across the link of the component of fluid flow in the direction of the link, usually referred to simply as the fluid velocity or fluid flow rate.
Representative of prior-art acoustic flow meters of the digital type is the one which is disclosed by Munston et al. in U.S. Pat. No. 3,894,431. In addition to a pair of transducers defining a communication link through a fluid the velocity of which is to be measured, the disclosed prior-art meter includes a pair of phase-locked loops. Each phase-locked loop has a voltage-controlled oscillator, referred to as a vco, frequency synchronized by the loop so as to generate a relatively high frequency signal that cycles n times during the period in which an acoustic wave propagates a respective direction across the link. Also included in the above-mentioned prior-art meter is control circuitry having a control oscillator for coordinating access to the link, and difference circuitry to combine the two vco generated signals to generate a difference signal having a frequency which represents the fluid velocity.
More particularly, in addition to the above-mentioned vco, each of the phase-locked loops includes an integrator for developing a vco-frequency-controlling potential and a transmit flip-flop reset by a cycle of the control oscillator and set by the cycle of the vco signal next following. The flip-flop develops a transmit-triggering signal marking the flip-flop-setting cycle of the vco.
Also reset by the flip-flop-resetting cycle of the control oscillator is a divide by n counter clocked by cycles of the vco signal. The counter develops a reference signal marking the nth vco cycle following the resetting cycle of the control oscillator. The transmit-triggering signal developed by the flip-flop and the nth cycle reference signal developed by the counter delineate an n-cycle reference period for comparison with the respective acoustic propagation period.
Also included in each phase-locked loop is a transmitter for exciting a respective one of the transducers to develop an acoustic-wave packet for propagation across the link and a receiver for amplifying a signal developed by the other one of the transducers responsive to the acoustic-wave packet transmitted across the link. The receiver has automatic-gain-controlling circuitry for controlling the gain of the receiver responsive to the level of the amplified signal and a zero-crossing-detector for developing a received signal responsive to a zero crossing within the packet of the amplified signal. The transmit signal and the received signal delineate a propagation period over a respective direction of the link.
In addition, each of the phase-locked-loops includes a phase detector for comparing the time marked by the nth cycle signal with that of the received signal to develop a complementary pair of error signals and a pair of monostable multivibrators driven by the error signals for appropriately incrementing or decrementing the integrator. The phase detector is of the non-linear, or bang-bang type, developing the error signals so as to indicate whether the nth cycle signal occurred before or after the received signal, in other words early or late. Responsive to each pair of early or late error signals, a respective one of the multi-vibrators develops a constant-width integrator-driving pulse for incrementing or decrementing by a fixed amount the charge stored by the integrator to increment or decrement slightly the vco controlling potential developed by the integrator thereby adjusting the operating frequency of the vco.
The control oscillator is operative to reset, in turn, each of the phase-locked loops thereby coordinating access to the link. In addition to the control oscillator, the control circuitry includes a delay circuit driven by the control oscillator and a strobe circuit driven by the delay circuit, the delay and strobe circuits for developing a receiver-enabling window signal for reducing the susceptibility of the receiver to noise.
Unfortunately, the circuitry employed by digital-type acoustic flow meters is relatively complex and expensive. Also, as a result of the relatively high frequencies employed by the vcos and the switchings transients associated with the counters and other digital circuitry of digital-type acoustic flow meters, sensitivity limiting noise is generated.
Brown et al. in U.S. Pat. No. 3,981,191 discloses an acoustic flow meter which is more nearly analog in nature. Rather than employing the frequency of a signal generated by a suitably locked vco as a measure of the fluid velocity, the above-mentioned prior-art meter employs the vco controlling potential. This also permits a single time-multiplexed phase-locked loop to be employed. The phase-locked loop develops early and late error signals which are suitably combined in a pair of integrators to develop a first potential that represents the acoustic velocity in the fluid and a second potential that represents the velocity of the fluid. From the first and second potentials sum and different potentials are developed which are multiplexed to control a single vco to delineate a pair of reference periods for comparison with an upstream and a downstream propagation period.
More particularly, the phase-locked loop also includes a divide-by-four counter driven by the vco, a transmitter and a receiver both coupled by a switch to a pair of acoustic transducers, a comparator, error-decoding logic circuitry, a pair of integrators and an adder. Clocked by cycles of the vco, the counter develops a signal for controlling the state of the switch, a signal for triggering the transmitter and an nth cycle reference signal. Together, the transmitter-triggering signal and the nth-cycle reference signal delineate, at different times, a pair of reference periods.
The transmitter and the receiver are so coupled by the switch to the pair of transducers that the state of the switch controls the direction across the transducer-defined link that an acoustic-wave packet is transmitted. The transmit-triggering signal and the responsive receiver-developed signal together delineate a propagation period across a respective direction of the link.
The time marked by the nth-cycle reference signal is compared with respect to that of the received signal by the comparator which develops an error signal having but two states, specifically: received signal early and received signal late.
The received early signals are subtracted from the received late signals in a first one of the integrators to develop the first potential, which represents the average of the two propagation periods, in other words, the acoustic velocity in the fluid.
Next, the upstream and the downstream components of the early and late signals are separated by the error-decoding logic circuitry using the switch-state-controlling signal to develop four signals for driving the second integrator. The four separated signals include an early upstream signal, a late downstream signal, an early downstream signal and a late upstream signal, which are combined in a second integrator such that the former two signals are subtracted from the latter two signals. Responsive to the combined signals, the second integrator develops the second potential representing the velocity of the fluid.
Finally, the adder, time multiplexed by the switch-state-controlling signal, combines the first and the second potentials so as to develop the time multiplexed sum and difference potentials for controlling, in a time multiplexed fashion, the frequency of the vco.
It should be noted that the fluid velocity as measured by the two above-mentioned prior-art meters is independent of the acoustic velocity. Although each of the equations which relate a vco frequency to a respective propagation period, contain as a term in the numerator, the acoustic velocity, the acoustic-velocity terms cancel when these equations are combined to develop an equation for the flow velocity.
The accurate measurement of the velocity of heterogeneous fluids present special problems for acoustic-type flow meters. Of course, if two few acoustic-wave packets are received, flow measurement may become impractical. In addition, gas bubbles, particular matter, etc. can cause attenuation and scattering of the packets which so attenuates and/or alters the shape of the leading edge of packets so as to degradate the accuracy of fluid velocity measurements. Obviousy, the shape and amplitude of the acoustic-wave packet can effect period measurements where edge triggering is employed, even when combined with agc circuitry. Even with the use of simple zero crossing detectors, the change in shape of the leading edge of the acoustic wave can cause the zero-crossing detector to skip between adjacent cycles of the signal representing acoustic-wave packets, degradating the accuracy of the meter.